Halocarbons
The broad class of halocarbons consists of all molecules containing carbon and one or more of the following halogen atoms: fluorine, chlorine, bromine, and/or iodine. Halocarbons, as the term is used here, may also contain other chemical features such as hydrogen, oxygen, and/or nitrogen atoms; carbon-to-carbon multiple bonds; and aromatic rings.
Due to their generally low toxicities and low or non-existent flammability, one family of halocarbons--the chlorofluorocarbons (CFCs), which contain only carbon, chlorine, and fluorine atoms--has been used for many years as foam blowing agents. Methyl chloroform, CH.sub.3 CCl.sub.3, has also been widely used.
Foam Blowing Agents
The manufacture of polymer foams for insulation, cushioning, packaging and other uses requires gas or volatile liquid blowing agents to create bubbles or cells. There are two types of polymer foams-open-cell and closed cell. Open-cell foams, sometimes called flexible foams, are used almost solely for cushioning since their open-cell structure allows gases to escape and re-enter the foam, providing flexibility. Thus, blowing agents are released almost immediately following production of open-cell foams (Farmer, R. W., and Nelson, T. P., Control Technology Overview Report: CFC-11 Emissions From Flexible Polyurethane Foam Manufacturing, EPA-600/2-88-004, U.S. Environmental Protection Agency, Research Triangle Park, N.C., Radian Corporation, Austin, Tex., January 1988). Closed-cell foams, on the other hand, have closed cells to provide rigidity. For many closed-cell foams, release of trapped gases is very slow and can be hundreds of years (Wert, K. P., Nelson, T. P., and Quass, J. D., Control Technology Overview Report: CFC Emissions from Rigid Foam Manufacturing, EPA-600/2-88-003, Environmental Protection Agency, Research Triangle Park, N.C., Radian Corporation, Austin, Tex., Jan. 1988). Closed-cell foams have a variety of uses, but are widely used as insulating materials.
CFC-11 (trichlorofluoromethane, CCl.sub.3 F), CFC-12 (dichlorodifluoromethane, CCl.sub.2 F.sub.2), CFC-113 (1,1,2-trichloro-1,2,2-trifluoroethane, CCl.sub.2 FCClF.sub.2), CFC-114 (1,2-dichloro-1,1,2,2-tetrafluoroethane, CClF.sub.2 CClF.sub.2), methyl chloroform, and mixtures of these chemicals are the most common materials that have been used in the past as blowing agents in the manufacture of polymer foam products. In addition to their remarkably low toxicities and lack of flammability, these materials provide closed-cell foams with excellent insulating ability and generally have good materials compatibility. The blowing agents may be used in a number of ways. For example, they may be mixed with the reactants forming a polymer and vaporized by the heat of reaction during polymerization to give a foam, as is normally the case with polyurethane foams. They may be injected into molten plastics under pressure and form foams upon expansion, as is done for extruded polystyrene. The blowing agents may also be incorporated into plastic beads, which expand upon heating, as in expandable polystyrene. Any other method permitting volatilization of the blowing agents to expand a polymer would also be suitable.
Global Environmental Problems
CFCs, and many other halocarbons, have come to be recognized as serious global environmental threats due to their ability to cause stratospheric ozone depletion and global warming and their significant atmospheric lifetime. The ozone depletion and global warming impact of chemicals such as these is measured by the ozone depletion potential (ODP) and global warming potential (GWP). ODP and GWP give the relative ability of a chemical to deplete stratospheric ozone or to cause global warming on a per-pound-released basis. ODP and GWP are usually calculated relative to a reference compound (usually CFC-11 for ODP and either CFC-11 or carbon dioxide for GWP) and are usually calculated based on a release at the earth's surface. It is important to note that ODP and GWP values must be calculated by computer models; they cannot be measured. As models, theory, and input parameters change, the calculated values vary. For that reason, many different values of ODP and GWP are often found in the literature for the same compound. Nevertheless, the calculation results are very accurate in predicting which compounds are highly detrimental to ozone depletion or global warming, which are only moderately detrimental, and which have very low or essentially zero impacts.
Despite the wide utility of CFCs, their production has been severely restricted due to concerns about stratospheric ozone depletion. In fact, under the Montreal Protocol, an international treaty enacted in 1987 and amended in 1990, 1992, and 1995, the production of CFCs was phased out in all industrialized nations at the end of 1995. Moreover, the production of certain other halocarbon chemicals has also been halted. Thus, the production of methyl chloroform (1,1,1-trichloroethane, CH3CCl3), which like CFC-113 has been widely employed as a foam blowing agent, was also ended at the end of 1995 in industrialized countries. Replacements and Proposed Replacements for Ozone Depleting
Chemicals
Among the earliest replacement chemicals proposed as replacements for CFCs and methyl chloroform were the hydrochlorofluorocarbons (HCFCs). These compounds contain hydrogen in addition to carbon, fluorine, and chlorine. The hydrogen atoms in the HCFCs react with hydroxyl free radicals, which are normal constituents of the earth's atmosphere, and, therefore, decrease the atmospheric lifetime of HCFCs relative to CFCs. This decrease in atmospheric lifetime limits the amounts of HCFCs that reach the stratosphere to deplete ozone.
HCFC-123 (2,2-dichloro-1,1,1-trifluoroethane, CHCl.sub.2 CF.sub.3), HCFC-141b (1,1-dichloro-1-fluoroethane, CH.sub.3 CCl.sub.2 F) and HCFC-124 (2-chloro-1,1,1,2-tetrafluoroethane, CHClFCF.sub.3) have been used or are planned for use as foam blowing agents to replace CFCs and methyl chloroform. Unfortunately, the atmospheric destruction process for HCFCs is insufficiently efficient to prevent all of the chemicals from reaching the stratosphere.
Thus, HCFCs exhibit a low, but significant, ODP. For that reason, HCFCs are scheduled for eventual phaseout under the amended Montreal Protocol.
Much research has gone on to find replacements for the CFCs, HCFCs, and methyl chloroform. Hydrofluorocarbons (HFCs), which contain only hydrogen, fluorine, and carbon, and perfluorocarbons (PFCs or FCs), which contain only fluorine and carbon, are being commercialized as replacement chemicals in a number of applications. Since these materials contain no chlorine, bromine, or iodine (which have been considered to be the cause of significant stratospheric ozone depletion), they have a nominally zero ODP. (Here, I use the word "nominally" since calculations have shown an exceedingly small ODP for some of these materials.) However, HFCs and PFCs have very long atmospheric lifetimes and contribute to global warming. In fact, many PFCs have atmospheric lifetimes of several thousand years, compared with a few years for most HCFCs and a few hundred years for most CFCs. Moreover, a number of HFCs and HCFCs are flammable.
Many non-halocarbons are being commercialized or seriously considered as CFC replacements. The hydrocarbon cyclopentane (C.sub.5 H.sub.10) is now used to blow refrigerator insulating foams in some parts of the world, and hydrocarbons such as n-pentane (CH.sub.3 CH.sub.2 CH.sub.2 CH.sub.2 CH.sub.3), isopentane [(CH.sub.3).sub.2 CHCH.sub.2 CH.sub.3 ], n-butane (CH.sub.3 CH.sub.2 CH.sub.2 CH.sub.3), and isobutane have long been used in the production of extruded polystyrene foam sheet products. However, these hydrocarbons are flammable. Other flammable chemicals being considered or being used as blowing agents are HCFC-141b, HFC-152a, 2-chloropropane (CH.sub.3 CHClCH.sub.3), and acetone [CH.sub.3 C(O)CH.sub.3 ]. Conversion from CFC and methyl chloroform to flammable blowing agents will entail significant capital investment to ensure worker safety. There is also concern about the ability of foams blown with flammable blowing agents to meet code requirements and safety standards. In fact, there has even been some flammability problems with foams blown with CFCs and methyl chloroform and often flame retardants are added to the foam to limit flammability. Such retardants can, however, degrade polymer properties.
Solution to Flammability Problems
Although methyl chloroform and many CFCs, HCFCs, HFCs, etc. are good blowing agents, the foams produced may still have some flammability problems due to the foam itself burning. With the new blowing agents, many of which are flammable, the problem of foam flammability may increase. Often additives are added to the polymers to reduce foam flammability. However, the use of additives as flame retardants has disadvantages. Such additives often have to be used in relatively high concentrations (typically 10 to 40 percent by weight) to be effective, leading to undesirable changes in physical and mechanical properties (Ebdon, J. R., Joseph, P., Hunt, B. J., Price, D., Milnes, G. J., and Gao, F, "Flame Retardance in Styrenic and Acrylic Polymers with Covalently-Bound Phosphorus-Containing Groups," BCC Conference on Flame Retardancy, Stamford Conn., Jun. 2-4, 1997). Moreover, flammable blowing agents cause safety problems during utilization. What I claim here is the use of specialized, tropodegradable blowing agents to (1) decrease the flammability of closed-cell blown foams and (2) decrease safety risks due to flammability during production of both open-cell and closed-cell foams. In the case of flammability reduction for blown closed-cell foams, the flame suppressing agent is released (from the foam) to extinguish the fire. The flammability reduction action of these agents is not due only to their heat absorption (a mechanism which is relatively ineffective), these agents also act chemically to actually suppress flammability. The mode of action is described immediately below.
Bromine- and iodine-containing compounds disrupt the free-radical chain reactions that maintain combustion. This disruption is a highly effective "chemical" mechanism for fire suppression, as opposed to the primarily "physical" mechanisms of cooling and smothering provided by nonflammable components used to obtain many nonflammable refrigerant and other blends. Iodides, though useful in direct fire protection technologies, appear to have too high a toxicity and too low a stability for serious consideration as agents in the specific applications discussed here. Bromine-containing compounds, such as the halon fire extinguishing agents, are also highly effective chemical fire suppressants. However, bromine-containing compounds in the specific chemical forms used today as fire extinguishing agents (primarily bromofluoroalkanes and bromochlorofluoroalkanes) have high ODPs because of their long atmospheric lifetimes, and their production has been banned in industrialized nations. Moreover, production of the one (briefly) commercialized bromine-containing halon replacement CHBrF.sub.2 (HBFC-22B1) has now also been banned in industrialized nations under the Montreal Protocol along with all other hydrobromofluorocarbons (HBFCs). In this case, the presence of a hydrogen atom in the molecule (without other features described in the present disclosure) was insufficient to achieve the hoped-for low atmospheric lifetime. In fact, none of the many halon substitute technologies now being commercialized contain bromine due to the concern about their expected high ODP. It should be noted that once they enter the stratosphere, bromine-containing compounds are about 40 times more destructive to stratospheric ozone than are chlorine-containing compounds (Solomon, S., and Albritton, D. L., "Time-Dependent Ozone Depletion Potentials for Short- and Long-Term Forecasts," Nature, Vol. 357, pp. 33-37, May 7, 1992).
The novel aspect of the present invention is the selection of bromine-containing compounds that solve the problem of stratospheric ozone depletion so that they can be used as blowing agents. If chemical features that promote extremely rapid atmospheric removal are incorporated into the compounds, insufficient amounts of the materials will reach the stratosphere to cause significant stratospheric ozone depletion. Thus, the compounds will have exceptionally low ODPs, even though they contain bromine, which is normally a strong ozone depleter. In fact, the resulting short atmospheric lifetimes will also result in low GWPs. Using this concept, I have (1) examined mechanisms for removal of compounds from the atmosphere, (2) determined chemical features that could enhance the various removal processes, and (3) carried out calculations to estimate the atmospheric lifetimes. This three-step process has allowed me to invent several families of bromine-containing halocarbons that have very short atmospheric lifetimes. Moreover, my calculations and estimation methods indicated that these compounds had much shorter atmospheric lifetimes than I had expected and that these very short atmospheric lifetimes resulted in very low estimated ODPs. I then discovered that such compounds can be used as agents to blow foams safely and to provide closed-cell foams having reduced flammabilities.